Durability of Japanese-earth cob walls subjected to accelerated rain simulation

Abstract Earthen materials are increasingly being recognized in architecture for their low embodied energy, recyclability, and hygrothermal properties. However, the common use of manufactured stabilizers, while enhancing weathering resistance, compromises these merits. Japan, known for its typhoons, has used unstabilized earth in construction for centuries, suggesting the viability of this construction material. While architecture around the world attests to its resilience, in-depth research into unstabilized earthen material is limited. This study examines unstabilized earth durability through 90-min accelerated rain simulation tests, totaling over 6500 mm of “rain” exposure on each of twelve test surfaces (eight representing rural Japanese cob-ball construction and four, monolithic cob walls). Surface changes were monitored by 3D scans performed at seven intervals. The test walls were built using two common materials in Japanese earthen wall craft, sourced from the areas of Kashiba and Fukakusa. It was clear from 3D-scan analysis that the base-layer material, Kashiba, is remarkably resistant to weathering. The four monolithic Kashiba test surfaces, constructed on four different foundation types, respectively eroded just 1 mm, 2.6 mm, 3.8 mm, and 3.9 mm. These results corroborate traditional building practices. Thus, our study also underscores the value of incorporating knowledge from vernacular earthen architecture professionals in expediting academic research. Additionally, our results suggest the potential of hose-showerhead tool use for research, and for on-site testing of earthen material erosion levels for material adequacy.